The invention relates to apparatus in which a sheet of thermoplastic material is softened and formed in a mould usually with the aid of a vacuum and/or fluid under pressure.
It is known to provide a single station thermoforming machine in which plastics sheet is loaded into the machine at the single working station and in which heaters can be moved into and out of the station to heat and soften the sheet while it is firmly clamped along all four edges in the station, the softened sheet subsequently being formed in the station and, after cooling, removed from the machine. With such a single station machine, because the sheet can be firmly clamped along all four edges and because it is possible to make the mould box an air tight chamber closed by the plastics sheet, it is possible to control sag in the sheet by adjusting the air pressure in the chamber. A single station machine is thus versatile in that it can cope with any mouldable thermoplastics material but it suffers from the disadvantage that it is slow in operation.
To speed-up the production cycle various high speed thermoforming machines have been proposed. One known high speed machine is a "shuttle" type machine which comprises a central forming station having heating stations on opposite sides of the forming station.
With such a machine, while one sheet is being heated in one of the heating stations, another sheet can be formed, unloaded and a fresh sheet loaded in the forming station. With a shuttle-type machine, not only is it the case that the heaters can only operate alternately so that they are idle for half of their working life but also it is the case that the speed of the machine is still usually governed by the period of time necessary to heat the sheet.
Rotary thermoforming machines are also known, and a three station rotary machine comprises a loading and unloading station, a heating station and forming station.
Since the sheet heating time is still critical with such a machine it is also known to provide a four station rotary machine, the extra station being a second sheet heating station. Four station rotary machines are relatively fast in operation but are complicated and thus expensive and are difficult to automate.
The extra expense is accounted for, at least in part, by the fact that each station must have its own sheet clamping frame and the difficulty in automation arises at least partly in the loading of sheet and unloading of the finished article. Normally such machines are loaded and unloaded manually.
To avoid at least some of the problems of these known machines it has been proposed to provide a so-called "in-line" machine in which the sheet is fed successively between loading, heating, forming and unloading stations arranged in a rectilinear array. Such a machine is capable of rapid operation since it can have as many heating stations as are required and of automatic loading and unloading but suffers from the disadvantage that hitherto it has not proved practical to maintain the sheet clamped on all four sides during its heating and during its transfer from the heating station to the forming station. Although it is theoretically possible to have an in-line machine consisting of a series of sheet clamping frames travelling in a closed loop, such a machine would be excessively expensive due to the large number of clamping frames which would be required. Also the time taken to adjust the clamping frames to cater for different sizes of plastics sheet would be unacceptably increased.
During the transfer the softened sheet tends to distort both due to sagging under gravity and/or due to an inherent tendency of plastics sheet to change shape when heated. As a result the conventional practice with in-line machines is to start with sheets which in their cold state are considerably larger than the finished article to allow for the shrinkage. While this was acceptable when the cost of plastics material was relatively low it is becoming increasingly less acceptable as time goes by especially since most plastics are based on oil.
At the heating station it is conventional to provide upper and lower heaters for example, infra-red quartz electric heaters which apply heat to the sheet until the plastics softens. It is known for the elements to be arranged in arrays on either side of the sheet and powered so that a predetermined pattern of heat is applied to the sheet in accordance with the shape of the article to be formed; thus where greatest forming of the thermoplastic material is required, these areas will be heated most and conversely for areas where little or no forming is required. This depends on each heating element being set to give a preset power output in accordance with the heating pattern to be applied to the thermoplastic sheet. With such preset power outputs heating of the thermoplastic sheet must be such as to avoid overheating the sheet locally and when thick thermoplastic sheets require to be formed, the time taken to heat the sheet determines the rate at which the thermoformings are produced. This limitation may therefore result in a higher cost per article produced.
It is furthermore a disadvantage of some plastics materials that they possess a very low hot strength and thus are difficult or impossible to thermoform on conventional high speed in-line apparatus, since during the transfer of the sheet it will tend to collapse.
It is an object of the invention to provide improved thermoforming apparatus.